This invention relates generally to amide polymerizations. More specifically, this invention relates to the recovery and recycle of unreacted amide monomers and oligomers in .epsilon.-caprolactam polymerizations.
In the polymerization of .epsilon.-caprolactam, about 10% of the caprolactam monomer remains unreacted. In addition, about 2.5% total cyclic oligomers form. The art has recognized the need to recover and recycle these valuable raw materials. Recovered materials are useful in later polymerization reactions or for other purposes. Further, if left in the polycaprolactam product, residual monomer and oligomers cause undesirable effects in further processing of the polymer product.
A typical recovery process involves hot water extraction of residual monomer and oligomers and recovery of the extractables by evaporating the water. In many cases, a first distillation step isolates the caprolactam monomer while the oligomer is depolymerized and distilled in later steps.
U.S. Pat. No. 4,053,457 to Cordes et al. describes caprolactam recovery. Cordes et al. discloses a process for extracting .epsilon.-caprolactam and other polyamide-forming starting materials. The extract containing solvent, monomer and oligomers is concentrated in the absence of atmospheric oxygen, the surfaces which come into contact with the extract being made of materials which are inert under the conditions of the concentration process, and the concentrate, without further purification or separation, is polymerized by itself or together with other polyamide-forming compounds.
U.S. Pat. No. 4,540,774 to Gerking et al. discloses a process for continuously demonomerizing and postpolymerizing polycaprolactam melts in a reactor designed for carrying out the process. Demonomerization is by vacuum. Condensed monomers and oligomers may be recycled, in whole or part, to the process origin.
U.S. Pat. No. 4,816,557 to Pipper et al. discloses a process for removing caprolactam monomer and oligomers thereof from nylon granules wherein the nylon granules are heated and superheated steam is passed therethrough in a treatment zone. The steam containing caprolactam and its oligomers is withdrawn from the top of the treatment zone. This steam then passes through a column to yield an aqueous solution of caprolactam and oligomers thereof and caprolactam free steam. According to the disclosure, the steam may be recycled.
As noted, it is known that recycling monomers left unreacted in polymerization reactions is advantageous. For instance, recycling monomer to fresh polymerizations preserves raw materials. In large volume operations, the conservation of raw materials can result in considerable cost savings. Other factors may merit consideration, too. As an example, recycling raw materials avoids the problem of waste disposal, a growing environmental concern.
For example, U.S. Pat. No. 4,537,949 to Schmidt et al. describes a continuous process for preparing certain nylons wherein prepolymer and vapors are continuously separated, the vapors are rectified and the entrained diamines are recycled. Although the Schmidt et al. disclosure does not specifically address .epsilon.-caprolactam monomers, the teachings recited highlight the importance given to recycling in general.
U.S. Pat. No. 4,429,107 to Strehler et al. discloses a process for continuously preparing polycaprolactam in which .epsilon.-caprolactam is partially polymerized, with the addition of a water-containing agent and acetic acid or propionic acid as a chain regulator, at a nylon-forming temperature. A gaseous mixture of caprolactam, water and acetic or propionic acid obtained at the top of the reactor is fed to the middle of a column. Water is removed at the top of the column and the bottom of the column is maintained at 125.degree. C. to 145.degree. C. The mixture obtained at the bottom of the column contains caprolactam, acetic or propionic acid and a small amount of water and is recycled to the top of the reactor.
U.S. Pat. No. 4,327,208 to Lehr et al. discloses a process for producing polyamide-6 or corresponding copolyamides by hydrolytic polymerization wherein the low molecular weight secondary reaction products and the unreacted .epsilon.-caprolactam are separated from the polyamide melt and directly condensed on an .epsilon.-caprolactam melt intended for polymerization. Reduced pressure and elevated temperature result in the separation of the monomers and oligomers from the polyamide melt. No intermediate treatment of the separated material takes place between separation and condensation. Lehr et al. also discusses the significance of cyclic dimer formation in the polymerization of .epsilon.-caprolactam.
Cyclic dimer forms during the initial ring opening of .epsilon.-caprolactam in the polymerization. The cyclic dimer is relatively stable and has a high melting point and low solubility. Moreover, cyclic dimer is not adequately processable at conditions and parameters suitable for polycaprolactam. One of the problems associated with cyclic dimer is its deposition on processing machinery which causes downtime in the mill for equipment cleaning. Much attention has focused on removing cyclic dimer from polyamides, particularly polycaprolactam.
Exemplary is U.S. Pat. No. 4,310,659 to Yates et al. describing a polymerization of .epsilon.-caprolactam which uses a two stage hydrolysis process. The first stage operates at a first temperature and a first pressure. Before equilibrium conditions are reached a second stage operates at a second temperature and pressure while water is continuously removed. As a result, water (containing hydrophilic extractables) is removed both during hydrolyzation and during the subsequent polycondensation so that the cyclic dimer content of the shaped polymer article is below 0.2 percent by weight.
U.S. Pat. No. 4,436,897 to Strehler et al. discloses a process for preparing polycaprolactam by polymerizing .epsilon.-caprolactam, and an aqueous extract containing .epsilon.-caprolactam and caprolactam oligomers. The extract has been obtained by extracting polycaprolactam with water sufficient to achieve an extract containing from 0.1 to 5.0% by weight of oligomers of caprolactam, based on the monomeric caprolactam in the aqueous extract. This extract is then used in later polymerizations. As disclosed, after the above extraction, a second extraction yields residual extract with a higher dimer concentration.
What remains needed is a method for continuously recycling wash water concentrate from amide polymerizations to the reaction vessel in the same proportion as extractables were removed without dilution with virgin monomer prior to concentrating while establishing a cyclic dimer concentration, relative to the reactants, well below its solubility concentration.